# Flex SDK Versions
The Flex SDK was open sourced as of version 3.
Version | Last Milestone or Stable Release (download)
Flex 4.6 | [November 29, 2011](Download%20Flex%204.6)
Flex 4.5.1 | [June 19, 2011](Download%20Flex%204.5)
Flex 4.5 | [May 2, 2011](Download%20Flex%204.5)
Flex 4 | [March 21, 2010 ](Download%20Flex%204)
Flex 3 | [December 11, 2009](Download%20Flex%203)
## Flex 4.6
Flex 4.6 is the next major step in cross-platform mobile application development and offers several new components and functionality specifically designed to help developers create applications for the latest mobile and tablet devices.
## Flex 4.5
The 3 primary themes of Flex's next release, code-named, Hero, are:
* **Multi-Screen Development**: Allow developers to build applications that target the web, desktop or mobile devices using a single unified framework.
* **Spark Maturation**: Polish and grow the Spark architecture by adding new Spark components and capabilities.
* **Large-Application Development**: Support developers building large applications by improving fundamental pieces of the Flex infrastructure.
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## Flex 4
As of March 21, 2010, Flex 4 has been released. It is has 3 primary themes:
* **Design in Mind**: provide a framework meant for continuous collaboration between designer and developer.
* **Developer Productivity**: improve compiler performance and add productivity enhancements to language features like data binding
* **Framework Evolution**: take advantage of new Flash Player capabilities and add features required by common use-cases
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## Flex 3
Flex 3 was initially released on February 25, 2008. It was focused in 4 areas:
* **Designer/Developer workflow**: e.g., enhanced constraints, Flex Component Kit for Flash CS3
* **Working with data**: e.g., list effects
* **Adobe AIR applications**: support in the framework for Adobe AIR features
* **Platform evolution**: e.g., persistent framework caching, compiler performance
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